This invention relates to the field of plasma processing, which field comprises Physical Vapor Deposition (PVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), film etching and patterning, surface modification, ashing, desmearing, and other plasma processes used in industry and research. Specifically, the invention relates to the narrower aspect of igniting a plasma in a processing system within which thin film processing is affected by the plasma.
To begin plasma processing one must create a plasma, which is a diffuse mixture of ions and electrons, usually (but not necessarily) at a pressure less than that of a standard atmosphere. The plasma is sustained by a variety of physical mechanisms, but to begin one must create an excess of electrons or ions. In most cases the sustaining mechanism then puts energy into the system to allow the charged particles to ionize other atoms and thus build the plasma density up to a working level.
As but one example, one might examine the plasma deposition system which uses a process called sputtering, a form of Physical Vapor Deposition (PVD). A common system for accomplishing sputtering uses a source which may provide a magnetic trap for electrons, and some power supply or means for pumping energy into the plasma through an electric field created by placing a voltage on an element called a target. This type of system, called a magnetron, is very commonly used to create and maintain plasmas for the purpose of sputtering deposition of thin films of metals and dielectrics onto appropriate substrates.
Such a system is usually thought of as being limited to certain pressure ranges for several reasons. In particular, at the lower end of the pressure range it becomes difficult to ignite the plasma, even though a plasma, once ignited, may be sustained and utilized for sputtering. It is often desirable to operate at these lower pressures because less of the background gas will be included in the growing film, gas phase scattering of the deposition stream is reduced, and unwanted chemical reactions with the background gas are lessened, among other reasons. In operating in the pressure region where ignition is difficult or even impossible, several approaches have been taken in the prior art to aid in starting the plasma. First, a pressure burst may be introduced into the system. That is, the pressure control system is caused to increase the pressure to a level at which plasma ignition can be achieved and may even be reliable and easy, and then the pressure is lowered again to the working level. Alternatively, auxiliary sources of ions or electrons may be used to increase the charge and therefore the probability of plasma ignition. These have included adding sources of ionizing radiation (radioactive sources), supplying high frequency power to the target or an auxiliary electrode, adding hollow cathodes or thermionic filaments to the system, or other sources of ions or electrons.
All of these methods add complexity to the system and may change the operating characteristics of the process so that non-uniform or unstable results may be obtained. Yet there has been a long term desire to operate at the lower pressures and this has driven workers in the field to use these methods in spite of their disadvantages.
In addition, many thin film processing applications involve the precise processing of one or more items in a very dynamic environment. This dynamic environment is not only unpredictable, it may cause fluctuations in a variety of characteristics necessary to cause the gas involved to ignite into a plasma. Thus, while in some other applications the plasma can be ignited by merely achieving the necessary conditions, in thin film processing applications, this is not always possible. The dynamic nature of the chamber itself and its unpredictability, may make knowing the conditions necessary for ignition extremely difficult from a practical perspective. Thus approaches such as that disclosed in U.S. Pat. No. 5,288,971 (hereby incorporated by reference) developed by Advanced Energy Industries, Inc. (the assignee of the present invention) have been created. This approach, while useful for many applications does not fully address other problematic applications.
As in the situation faced by the aforementioned prior art, two other aspects exist which explain the difficulty those attempting to ignite a plasma for thin film processing have faced. First, as mentioned, the processing is often very delicate. As semi-conductor manufacturing techniques have been refined, the thickness of the layers within them has decreased to the atomic scale. Thus, tolerances within the processing itself have become much more exacting. While at first glance this would appear to relate only to the operation of the plasma, not to its ignition, such is not the case as the environment during ignition itself can literally ruin the processing desired. Prior to the present invention, however, a technique for igniting a plasma in a processing system which both worked for the types of applications addressed here and which appropriately minimized undesirable effects has not existed.
As before, ignition (as well as processing itself) can be considered to be an eclectic art in some respects. While on the one hand the physical and chemical processes involved during thin film processing have been studied and refined from very theoretical bases to produce state of the art devices, still, oftentimes, some of the equipment utilized to actually achieve the thin film processing and the techniques which have been applied to achieve ignition might even be characterized as developed on a trial-and-error basis. It may even be said that while those skilled in the art of thin film processing might typically have a great degree of skill and technical knowledge in the plasma processes themselves, those so skilled rarely combined such knowledge with equivalent understanding of process and mechanisms involved in the ignition of the plasma or the electrical circuitry characteristics as well.
Prior to the present invention, several other techniques were known to ignite plasmas in general. Primarily, these techniques included the addition of separate equipment in order to achieve the conditions necessary to ignite the gas into a plasma. An example of these is U.S. Pat. No. 4,906,811 which discloses the addition of separate equipment to locally increase pressure in the gas to effect conditions more conducive to the ignition of the gas into a plasma. Different techniques similarly add separate equipment to create other such drastic changes. U.S. Pat. No. 4,557,819 discloses a separate DC voltage source to momentarily increase the voltage across the gas by about 5,000 volts; U.S. Pat. No. 4,888,088 discloses a separate, lower frequency power source to achieve ignition; and finally, U.S. Pat. No. 4,859,909 discloses the utilization of ion sources or electron sources to prompt the ignition of the gas into a plasma. As mentioned, each of these utilizes separate equipment to effect gross changes under which ignition of the plasma is almost certain to occur. The techniques used however, have not been accomplished in a manner which demonstrates the type of understanding of the environment as disclosed here.
Accordingly, the present invention is directed to a system for enhancing plasma ignition through manipulation of the voltage applied to the plasma electrodes in a manner which achieves advantages compared to those approaches previously used. This system is designed to achieve a manipulation so that a rapidly rising voltage waveform may act to concentrate free ions and thus increase the probability and variability of conditions which permit plasma ignition. The present invention discloses techniques which now achieve or perhaps more reliably accomplish ignition of a plasma in a thin film processing system. By making what might be seemingly subtle changes to the way in which existing equipment is operated, significant increases in the likelihood of ignition may often be achieved.
Accordingly a general object of the present invention is to achieve ignition without additional equipment. In keeping with this goal, the invention teaches the uses of operating equipment in a new manner for ignition purposes. As one embodiment, it is the goal of the invention to manipulate the operating equipment in a fashion which is likely to quickly and reliably ignite the plasma.
It is also an object of the present invention to permit plasma ignition at a variety of conditions especially at a lower pressure than has been heretofore possible. Similarly, it is an object of the present invention to permit more reliable plasma ignition at low operating pressures. It is thus a goal to permit more variation in the pressures possible for an plasma ignition environment and to permit ignition at pressures desired to be used in certain operating conditions.
It is a further object of the present invention to minimize or perhaps eliminate the utilization of other techniques to achieve plasma ignition. As one example, it is an object to perhaps eliminate any need for a change in pressure to effect plasma ignition. Another broad object of the present invention is to minimize the impact of ignition within a processing system. An object is thus to allow ignition while avoiding separate inputs to the processing environment.
It is another object of the present invention to avoid or minimize any requirement for additional components or systems with such a system. As one example, it is an object to eliminate any need to include sources of ions or electrons to effect plasma ignition at low pressures.
As in earlier inventions, another broad object of the invention is to establish a new type of fundamental criteria which can then be varied to create a variety of techniques to achieve reliable ignition.
Naturally, further objects of the invention are disclosed throughout other areas of the specification and claims. Certainly, other objects and advantages of the present invention are apparent from a reading of the specification.